In order to realize a smaller, thinner, and more reliable optical pickup device, such an optical pickup device that uses a hologram has been devised. A basic configuration of such an optical pickup device for DVDs (Digital Video Disks) is described in Japanese Publication for Unexamined Patent Application, Tokukaihei 9-161282 (publication date: Jun. 20, 1997).
The hologram described in the foregoing Publication is divided into two portions in a disk radial direction, and one of the two portions is further divided into two portions in a track direction. A half of a reflected beam from the disk is used to detect a focus error signal, and the other half of the reflected beam is used to detect a track error signal. The reflected beam as a whole is used to detect an information signal. According to this configuration, a positional signal for a track, known as a push-pull signal (PP signal), can be detected from the track error signal, by further dividing a radially divided half of the beam into two in the track direction of the disk.
The optical pickup device includes a light integration unit using the hologram of the foregoing configuration, and object lens means for focusing, onto the disk, a laser beam emitted from the light integration unit.
The pickup device using the light integration unit of the foregoing configuration has the following problems.
In generating the track error signal, a two-part detector detects a difference between light intensity distributions of a right portion and a left portion of the reflected light from the disk (a radially inner portion and a radially outer portion of the light beam divided by a divisional line in a track direction).
Here, in case the object lens shifts in the radial direction, an optical axis of the reflected light beam from the disk shifts, and the center of the beam deviates from the center of the two-part detector. The object lens is displaceable within a range of approximately ±0.3 mm, so as to accommodate disk decentering. Therefore, shifting of the object lens is also caused by disk decentering.
Likewise, the center of the reflected light beam also deviates when the disk is inclined. Therefore, in either case, detracking is caused by an offset in a differential signal of the two-part detector, even though tracking is proper.
In addition to the push-pull method (PP method), a three-beam method and a differential push-pull method (DPP method) are generally used as a tracking servo method.
All of these methods detect a detrack amount by detecting a difference in light quantity of a plurality of light receiving sections. If there is no difference in light quantity, it is judged that a just track is attained.
In the three-beam method, the beams include a main beam and sub beams in front of and behind the main beam. A track error is detected from a differential signal of the sub beams. The DPP method is a combination of the PP method using a single beam, and the three-beam method, whereby a track error is detected from a differential signal of the main beam and the sub beams divided in the track direction.
This makes it possible to suppress an offset caused in the PP method. Therefore, the DPP system is a commonly used tracking servo method.
However, in these methods, the three beams are generated by a single light source. This decreases a light quantity of the main beam used for recording. As a result, there is a problem that a recording speed slows, and a high-speed recording is hindered accordingly.
An object of the present invention is to provide a light integration unit, using the one-beam method, for obtaining stable tracking servo performance by suppressing an offset caused by shifting of the objective lens and tilting of the disk, without reducing the light quantity of a main beam despite the one-beam method, and to provide an optical pickup device and an optical disk device using such a light integration unit.